Various types of recognition systems are taught by the following commonly assigned U.S. patents and applications: U.S. Pat. Nos. 5,287,112; 5,270,717; 5,196,735; 5,170,493; 5,168,282; 5,126,745; 5,073,781; 5,053,774; 5,025,492; U.S. Ser. No. 08/021,123, filed Feb. 23, 1993 [TI-17529], now U.S. Pat. No. 5,701,127; Ser. No. 08/065,286, filed May 21, 1993 [TI-16981], now abandoned; and Ser. No. 08/086,786, filed Jul. 2, 1993 [TI-17507], U.S. Pat. No. 5,347,280. Systems conforming to the teachings of the foregoing documents are marketed under the name TIRIS ("Texas Instruments Register and Identification System"). A recognition system similar in result to, but structurally and functionally specifically different from, TIRIS is disclosed in U.S. Pat. No. 4,918,955. Other types of recognition systems include systems known as AVI (for "Automatic Vehicular Identification"), as taught, for example, by commonly assigned U.S. Pat. No. 5,287,112 and commonly assigned U.S. application Ser. No. 08/021,123, filed Feb. 23, 1993.
In one type of TIRIS recognition system of interest, a transmitter/receiver (also referred to herein, in the claims hereof and elsewhere as an "interrogator" or a "reader") selectively radiates energy via an associated first antenna. The energy is radiated from the vicinity of a key-operated lock, such as a vehicle ignition switch. The radiated energy is often referred to as an "interrogation signal." The radiated energy is received by a second antenna present on or in a key. The key also includes facilities, such as circuitry (sometimes called a "transponder" or "tag"), connected to the second antenna. An electrical signal produced or induced in the key's circuitry by the received energy either is affected (e.g., increased or decreased) by the circuitry or effects the generation of a stored, uniquely-coded signal by the circuitry. The affected or coded signal is often referred to as a "recognition signal."
Depending on the constituents of the key circuitry, either the recognition signal is transmitted or reflected back to the transmitter/receiver via the second and first inductors or antennas. A key "matching" the particular ignition switch transmits or causes a predetermined recognition signal. Other keys which operate other ignition switches associated with similar recognition systems may similarly respond to the interrogation signal but transmit recognition signals different from the predetermined recognition signal.
The transmitter/receiver includes facilities which analyze the recognition signals received thereat to determine if the analyzed signal is the predetermined recognition signal produced by the matching key. If the analyzed signal is the predetermined recognition signal, the concurrence of such predetermined recognition signal and the operation of the ignition switch by the matching key starts the engine of the vehicle. If the analyzed signal is not the predetermined recognition signal, either the ignition switch cannot be operated by the key, or, if it can be operated, such operation is ineffective to start the engine.
Portability and/or space limitations usually result in the transmitter/receiver of a TIRIS-type of recognition system being not very powerful. Also, the recognition signals, that is, the signals transmitted or reflected back to the transmitter/receiver from the key-included circuitry, may be derived from the limited energy radiated from the transmitter/receiver, not from energy derived from a key-contained power source, such as a battery, as is typical in systems of the AVI type. While the use of a battery with key-included circuitry of a TIRIS system is technically possible, the large size and resulting unwieldiness of the resulting key would probably lead to rejection by users. As a consequence of the foregoing, it is critical that circuit efficiencies be as high as possible.
One type of recognition circuitry of the subject type includes active and passive components, which in response to the receipt of energy from the transmitter/receiver produce a coded signal. See the above-noted commonly assigned U.S. patents and applications. The coded signal, which may be produced by data stored in memory, is transmitted back to the transmitter/receiver, where comparison with the stored "matching" signal is carried out. The coded signal may be produced by modulating a carrier with the stored code, and the carrier may be, or may be derived from, the energy received by the key-included facilities from the transmitter/receiver. In this latter event, the system may be of the TIRIS variety, and the key and its circuitry require no on-board power source and may be said to be "batteryless." This third type of system may also be of the AVI variety, in which case the transponder is typically powered by a self-contained power source.
Recognition systems of the above type may be the full-duplex variety. Specifically, the transmitter/receiver may simultaneously operate as both a transmitter and a receiver. That is, it may simultaneously radiate energy to the key-included circuitry and receive for analysis the corresponding signal produced by such circuitry. Typically, in full-duplex operation, the frequency of the modulated carrier radiated by the transmitter/receiver to the key-included circuitry is different from the frequency of the modulated carrier produced by the key-included circuitry and thereafter received and analyzed by the receiver/transmitter. See above-noted commonly assigned application Ser. No. 08/021,123 [TI-17529].
The full-duplex (FDX) transponder is continuously powered with RF energy by an interrogator unit and its antenna. The transponder receives the energy by means of a coil.
In operation, the full-duplex transponder receives energy from the interrogator unit via the electromagnetic energy that the interrogator antenna transmits and the transponder coils receive. In responding to the interrogator, the transponder retrieves information that a transponder memory device stores. The transponder modulates the information, using a damping modulation function and associated circuitry by applying a bit stream to a modulation circuit that modulates energy consumption through the transponder coil. For proper transponder operation at both far and near distances, the current from the damping modulation circuit should be as large as possible. At the same time, however, the transponder circuit operation requires at least a minimum amount of power from the received energy to properly operate.
Conventional damping modulation circuits suffer from several limitations. Known damping modulation circuits cannot react properly to fields of different strengths. On the one hand, if the fieldstrength of the transmitter signal is very low, such as in far distance cases, transistors within the damping modulation circuit do not conduct current and the coil voltages become less than the necessary voltage for proper transponder circuit operation. One the other hand, if the fieldstrength is too high, the same transistors are too weak and damping does not occur to a sufficient degree so that a proper modulation index results in the transponder circuit. This problem not only occurs when attempting far and near distance operations, but also if the reactive power in the inductive portions of the transponder circuit is excessively high or if the operating temperature of the transponder circuit, and, in particular, the damping modulation circuit varies significantly.